This invention relates to communication of data among global navigation satellite system (“GNSS”) receivers, virtual reference stations, data processing centers, rovers, and other apparatus, and in particular to techniques for transmitting that data in a format which conserves use of bandwidth available for the transmission of the data. Herein we describe the invention in terms of use with the US global positioning satellite system (“GPS”), but we use the terms GNSS and GPS interchangeably to refer to any global navigation satellite system, whether based on satellites provided by the United States, or by other countries, e.g. GLONASS (Russia), Galileo (European Union), GAGAN (India) etc. As will be apparent, the technology described herein can be applied to use with any GNSS system, including GPS.
A technique known as real time kinematic (RTK) positioning is now commonly used to determine the position of a roving GNSS receiver relative to a known reference point. This method requires that data from a reference receiver, or from a reference station network, be transmitted to the roving GNSS receiver in real time. The RTK technique is routinely used for surveying, mapping, and precise positioning applications in mining, construction, and many other industries. In RTK positioning, a set of GNSS receivers track GNSS signals from a constellation of satellites. In a typical application, a reference (or base) station is established at a known location such as a previously surveyed location, a benchmark, or other desired position. That reference station then provides GNSS measurements to a rover (or remote) station to enable the rover station to calculate its precise location relative to a previously determined reference point. The rover station, often carried by a person as a backpack or mounted on a moving machine, is then able to move about an area and use signals from the reference station to precisely locate desired positions within the area. Communication between the reference station and the rover is provided over a data link, for example, using private licensed radio, unlicensed radio, satellite, cellular, or other communication technology. At the rover the data is received and used for processing and/or display to the user of the system.
Allocation of radio, cellular, or other wireless frequency bands is commonly performed by a government organization. In the United States, the Federal Communications Commission determines appropriate allocation of the frequency spectrum, and similar organizations perform that service in other countries. Increasing demands for frequency allocation for new radio, cellular, and satellite applications, however, have made available bandwidth scarce. At the same time, the amount of data to be communicated among GNSS receivers continues to increase. It has therefore become more and more desirable to communicate that data using as little bandwidth as possible.
One prior art technique for communicating data among GNSS receivers was developed by the assignee of this invention, Trimble Navigation, and is commonly known as the “Compact Measurement Record” (CMR). A description of that technique can be found in N. Talbot, “Compact Data Transmission Standard for High-Precision GPS,” Proceedings of ION-GPS-96, Kansas City, Mo., 861-871 (1996). Although this format is now in widespread use, the format still requires a minimum data rate that will increase as more satellites are included in the available constellation of satellites desirable for use. To provide the service needed in real time means that more bandwidth will be required, and that bandwidth may not be available via the current crop of communications services. Most communications services have a fixed bandwidth available that cannot be exceeded. Accordingly, the present invention is directed to a technique for further reducing the amount of information required to be sent among GNSS receivers when bandwidth is at a premium, cannot be expanded, or in situations such as satellite transponders where expanded bandwidth is available only at higher cost.